1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus such as a copier, printer, a facsimile machine, a plotter, and a multi-functional system including a combination thereof, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, plotters, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charging device uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a pair of looped belts or rollers, one being heated by a heater for melting toner (hereinafter referred to as “fixing member”) and the other being pressed against the fixing member (hereinafter referred to as “pressing member”). In a fixing process, the fixing member and the pressing member meet and press against each other, forming a so-called a fixing nip through which a recording medium is passed to fix a toner image thereon under heat and pressure.
To extend product life of the fixing member, which is subjected to constant heat and pressure, changing the position of the pressing member is proposed. More specifically, the position of the pressing member is changed between a pressing state and a pressure-free state by rotating a cam so that the pressing member is not always in contact with the fixing member. The position of the pressing member is also changed deliberately to accommodate recording media sheets in different types and sizes. Further, the position of the pressing member is changed to change an extent of contact between the fixing member and the pressing member, that is, a width of the nip and a nip pressure in accordance with the recording medium fed to the nip.
Some fixing devices are provided with a sheet guide to guide the recording medium properly to the nip and a sheet separator to facilitate separation of the recording medium from the pressing member after the nip at a certain position. If not properly positioned, the recording medium fails to be conveyed properly to the nip or separated from the pressing member. In the fixing device using a movable pressing member to change the nip width and the nip pressure as described above, changes in the position of the pressing member complicate efforts to properly position the sheet guide and the sheet separator relative to the pressing member, causing possible failure in the fixing operation of the toner image and in conveyance of the recording medium.
In this type of the fixing device, the pressing member is heated by a heater, and the temperature of the pressing member is adjusted by detecting the temperature of a surface of the pressing member by a temperature detector. In this configuration, changes in the position of the pressing member causes misalignment of the pressing member relative to the temperature detector, complicating efforts to detect and adjust the temperature of the pressing member correctly, ultimately causing fixing failure. Similarly, when provided with a thermostat and/or a temperature fuse, changes in the position of the pressing member also cause misalignment of the thermostat and the fuse relative to the pressing member, hindering reliable prevention of overheating of the pressing member.